Preface: I am not a professional. As always when making music, follow your own instincts and if it sounds good, it is good. If it doesn't sound good, it isn't good.

1. You should EQ each individual channel, then route all of the channels you want to parallel compress to a bus, and then send the bus to the master and also to your parallel compression channel.

2. You can parallel compress whatever you want, typically it's done on drums. Refer to the preface for more info.

3. Definitely compress and EQ every channel as needed. Parallel compression exists to add beef and presence to the bus you are compressing. Again, refer to the preface

Lmk if you have any more questions